1. Field of the Invention
The present invention relates to a press-contact flat type semiconductor device, such as a center gate type thyristor.
2. Description of the Related Art
A press-contact flat type semiconductor device has increasingly been employed to control large power supply to various associated industries, such as vehicle industries, iron mills and welding machine industries. Of this type of semiconductor device, a GTO (gate turn-off thyristor) belonging to a center gate type thyristor will be explained below as being a conventional example.
FIG. 1 is a cross-sectional view showing a GTO of a conventional alloy junction type. In FIG. 1, respective component parts are shown separated away in a press direction for ease in viewing. A package comprises a bottom structure 1 and a top structure 2.
A copper cathode electrode post 1 is silver-brazed by a metal ring sheet (hereinafter referred to a ring) 1b to the bottom surface of an insulating (ceramics) cylinder 1a. A weld ring metal sheet (hereinafter referred to as a welding) 1c is silver-brazed to the top face of the ceramics cylinder la. A gate electrode connection gate pipe 1d is silver-brazed to the side surface of the ceramics cylinder 1a. The bottom structure 1 comprises the aforementioned members as identified by reference numerals 1 and 1a to 1d. The top structure 2 comprises an anode electrode post 2 made of copper and a weld ring 2a which is silver-brazed to the anode electrode post 2. Members as identified by reference numerals 3 to 20 are sealed within the package. The sealing is performed by welding together the weld rings 1c and 2a and completed by inserting a lead wire of a gate lead 9 through a gate pipe 1d and performing a welding with a N.sub.2 gas sealed in an inner space.
An anode electrode, not shown, is provided on that major surface (an upper surface) of a silicon pellet 16 in the GTO device which is located on the anode electrode post side. A cathode electrode and gate electrode are formed on that major surface (lower surface in FIG. 1) of the pellet 16 which is located on the cathode electrode post. The anode electrode on the upper surface of the silicon pellet 16 is initially alloy-bonded to a W disc (tungsten disc) 18 by means of an Al-Si sheet (brazing material) 17. A Si-pellet/W-disc bond unit may produce a warp caused by a difference in thermal expansion coefficient between the silicon and the tungsten and hence produce a defect upon the press contact of the associated components. For this reason, a sheet 19 of a soft metal, such as Ag, is sandwiched between the anode electrode post 2 and the tungsten disc 18 and silver caps 13 and 11 are located between the cathode electrode post 1 and cathode and gate electrodes of the silicon pellet 16. This arrangement assures a better press contact of these associated components.
A molybdenum foil 14 is disposed between the silver cap 13 and the aluminum cathode electrode on the silicon pellet 16 to prevent the silver cap from being attached to the cathode electrode. A cutout 21 is provided in the cathode electrode post 1 to allow the gate lead 9 to be taken out therethrough. The use of a somewhat thick molybdenum disc 12 ensures the uniform press contact of the silver cap 13 over the whole surface of the cap. Silicon rubber members 15 and 20 are employed to electrically insulate the anode electrode side of the silicon pellet 16 from the cathode electrode side of the silicon pellet 16. A steel sheet (1.5t) 6 electrically insulated by a mica 4, Teflon ring 5 and Teflon tube 10 from the cathode electrode post, a pair of disc springs 7, Mo sheet (0.3t) 8, gate lead 9 and Ag cap 11 are provided between the gate electrode situated at the central portion of the silicon pellet 16 and the cathode electrode post 1. The cathode electrode post 1 allows the Ag cap 11 to be pressed by the aforementioned associated members against the gate electrode of the silicon pellet 16 with a steel sheet 3 inserted as shown in FIG. 1.
The GTO device are used with the anode electrode post and cathode electrode post pressed against each other by external pressure applying means, not shown, in a face-to-face direction.
The aforementioned GTO device has drawbacks (a) to (e) as will be set forth below.
(a) The silicon pellet 16 is alloy-bonded to the tungsten disc 18 with the use of the Al-Si sheet 17, producing a warp on the bond unit. In order to obtain more uniform press contact at the time of press contact, the silver caps 11 and 13 and silver sheet 19, made of a expensive metal of silver, have to be used in the positions shown in FIG. 1.
(b) Since the silver pellet 16 and tungsten disc 18 are alloy-bonded to each other, more assembling steps are required than otherwise.
(c) It is necessary to employ the silver cap 13 as set forth below so as to avoid a possible warp at a bond unit between the silver pellet 16 and the tungsten disc 18. In this case, in order to prevent the cathode electrode (aluminum electrode) from being attached to the silver cap 13, it is necessary to insert the molybdenum foil 14 therebetween.
(d) A greater number of components has to be inserted between the anode electrode post 2 and the cathode electrode post 1 and most of them are expensive, such as members 12 to 19 in FIG. 1.
(e) The silicon rubbers 15 and 20, an insulating material for electrically insulating the anode side surface from the cathode side surface of the silicon pellet, are complex in configuration and numerous in number.